1. Field of the Invention
This invention relates to an integrated circuit structure and method of making that structure, and more particularly to a thin layer structure having ferroelectric properties.
2. Description of the Related Art
Ferroelectric materials are presently finding increased application in devices including non-volatile ferroelectric random access memories (NV-FcRAMs), uncooled infrared (IR) detectors, spatial light modulators, and microelectromechanical systems. Many of these applications require optimized ferroelectric properties. Due to the anisotropic nature of ferroelectric materials, such as Pb(Zr,Ti)O.sub.3 (PZT), orientation control may be used to produce thin films or layers with optimized electrical properties. Ferroelectric PZT exists in two forms at ambient temperatures: a tetragonal phase, in which the polar vector is aligned parallel to the [001] direction (or c-axis) and a rhombohedral phase in which the polar axis is aligned along the [111] direction. In the tetragonal phase, anisotropy exists in a number of electric properties, including dielectric constant, remanent polarization, and pyroelectric coefficient. Several applications benefit by manipulation of these properties. Uncooled IR detectors require a high pyroelectric coefficient (p) and low dielectric constant (.epsilon.) for maximum voltage responsivity, which is proportional to p/.epsilon..
In tetragonal materials, the maximum pyroelectric coefficient is found along the c-axis. Likewise, the minimum dielectric constant is also found along the c-axis of the crystal. It is thus highly advantageous to control orientation in a thin film or layer such that the c-axis is perpendicular to the plane of the film for a parallel plate capacitor geometry wherein the electrodes lie above and below the film and within parallel planes. In contrast, a film in which the a-axis is oriented perpendicular to the plane of the film for a parallel plate capacitor geometry results in low pyroelectric coefficient and high dielectric constant, which minimizes voltage responsivity. A [111] oriented tetragonal film represents an intermediate case because the average properties of the film can be expressed by resolving the anisotropy into the rectangular coordinates by simple vector algebra.
In rhombohedral ferroelectric PZT, maximum pyroelectric response can be attained in [111] oriented films. In another application, NV-FeRAMs require high remanent polarization to minimize performance requirements of sense amplifiers which read the stored charge. Furthermore, low switching voltages (i.e., low coercive fields) are useful to minimize power requirements for portable devices. In the PZT system, coercive field decreases with increasing Zr content, making high Zr compositions desirable. In this case, a [111] oriented rhombohedral PZT film maximizes the remanent polarization, and a [111] oriented tetragonal PZT film has a higher polarization than a [100] oriented tetragonal PZT film. This also occurs because the average properties of the film or layer can be expressed by resolving the anisotropy into the rectangular coordinates by simple vector algebra.
Various methods have been proposed for controlling the orientation of the crystal lattice structure of ferroelectric thin films. One method to control orientation is by utilizing substrate materials with a coefficient of thermal expansion (CTE) mismatched to that of the ferroelectric film. If the CTE of the substrate is higher than that of the film, the film will be in a state of compression on cooling through the Curie point, (i.e., the temperature where the crystalline phase transforms from the high temperature cubic (paraelectric) state to the low temperature tetragonal (ferroelectric) state. This situation results in a tetragonal film with a preferred [001] orientation. A substrate leading to this orientation must have a CTE larger than the CTE of PZT. MgO is such a substrate material.
A problem with this mismatched CTEs method of producing oriented tetragonal films is that silicon (Si) is the semiconducting substrate of choice for monolithic integration of ferroelectric material with integrated circuits. Silicon has a CTE that is less than that of PZT, so a PZT film deposited on a silicon substrate goes into tension on cooling through the Curie point, resulting in a highly undesirable [100] orientation.
Corresponding CTE issues are present for other substrate materials such as III-V materials, e.g., gallium arsenide and indium phosphide, or germanium. While crystal orientation may be controlled by using a substrate with a coefficient of thermal expansion (CTE) lower than the ferroelectric film, the present invention to control crystal orientation in ferroelectric thin films may in general have advantages for any substrate, since the bottom electrode is typically isolated from the semiconducting substrate by insulating layers that may have adhesion promoting or diffusion barrier properties. Substrates of the greatest technological interest are Si, GaAs, Ge, InP and any other semiconductor materials that would allow monolithic integration of the ferroelectric capacitor with transistors fabricated in the same substrate. Also of technological interest are substrates such as glass or ceramics, or metals, where the transistors reside in another substrate, and are connected to the ferroelectric capacitors in a hybrid configuration.
Another method for manipulating thin film orientation includes using seed layers. For example, in perovskite materials of formula ABO.sub.3, the orientation is very sensitive to changes in A-site/B-site ratio. Changing the Pb/Ti ratio in a thin film of PbTiO.sub.3 shifts the lattice orientation from [100] for Pb-rich films to [111] for Ti-rich films. See M. Shimizu, M. Sugiyama, H. Fujisawa, T. Hamano, T. Shiosaki, and K. Matsushige, "Effects of the Utilization of a Buffer Layer on the Growth of Pb(Zr,Ti)O.sub.3 Thin Films by Metalorganic Chemical Vapor Deposition, J. Crystal Growth, Vol. 145 (1994), pp. 226-231. The [111] oriented seed layer resulting from a low A/B site ratio is useful for depositing thereon a high Zr content ferroelectric film because the polar axis in the high Zr content ferroelectric film is also along the [111] axis.
Although PbTiO.sub.3 crystal orientation is influenced by the A-site/B-site ratio, PZT is not similarly influenced. There is a drawback from using the PbTiO.sub.3 seed layer even though it can control orientation of the ferroelectric PZT film. Unfortunately, the Ti-rich films, which result in the desired [111] lattice orientation, also have a potential to generate oxygen vacancies, as charge compensation for excess Ti. These oxygen vacancies cause undesirable electrical conduction in the perovskite film. Oxygen vacancies may also interact with domain walls to cause fatigue and imprint.
Thus there remains a problem of uncovering a process that will produce a perovskite film without the shortcomings of the prior art.